The Vatican wrote:The wine that is used in the most sacred celebration of the Eucharistic Sacrifice must be natural, from the fruit of the grape, pure and incorrupt, not mixed with other substances. […] Great care should be taken so that the wine intended for the celebration of the Eucharist is well conserved and has not soured. It is altogether forbidden to use wine of doubtful authenticity or provenance, for the Church requires certainty regarding the conditions necessary for the validity of the sacraments.

Is wine mixed with grape brandy allowed for these purposes? Perhaps a Port company, presumably but not necessarily a Portuguese company, could seek a ruling from the relevant ecclesiastical authorities?